The present invention pertains to an echography probe and an echograph especially designed for use in medicine.
Among the methods of investigation used to depict parts of the human body, echography has a special place. In particular, it is used to depict phenomena involving the flow of blood, often in the region of the heart. It is also used to depict cross-sections of organs located before a measuring probe. An echograph used to make echograms comprises means to produce a sound pulse and a probe by which body to be examined is subjected to the effects of this pulse. It also comprises a probe to receive the sound wave backscattered back to the body after this body has been thus excited. Processing means are used to measure the characteristics of the received signal. These processing means may be linked to storage means, or directly to imaging means to depict an image of the distribution of the measured characteristics. A special kind of measurement pertains to the characterisation of the tissues: this measurement consists in delivering, to each part of the body, a pulse with a sound absorption coefficient which is proper to the nature of the body at the part considered, because an ultrasonic wave is absorbed in varying degrees, depending on the nature of the body. The measurement of the absorption coefficient is therefore representative of the nature of the tissues which constitute the body.
The absorption coefficient of a tissue is expressed in decibels per centimeter and per megahertz. This coefficient, which is called .alpha. in specialist literature, expresses for example, the fact that an ultrasonic wave emitted at 10 MHz and penetrating to a depth of up to 5 centimeters inside the body (10 cm. back and forth) will have undergone an attenuation of 100.alpha. dB (100 dB if .alpha.=1) when coming out of the body. In other words, with a known working frequency and a known amplitude of received signals coming from two points at varying distances (the position of these points being known because the velocity of the sound wave in the body is known) it is possible to deduce the value of the coefficient .alpha. at an intermediate point between these two first points. For this measurement of the absorption coefficient, a measurement is made of the frequency alteration of the backscattered signal in a spectral band. Allowance is made for the fact that if the spectrum of the pulse which excites the body is Gaussian around a central frequency f.sub.0 , the spectra of the backscattered signals received, pertaining to those parts of the body which are located on either side of the point in question, are also Gaussian but with decreasing central frequencies, depending on whether the points are further away from or closer to the place of emission.
The alteration contributed by the body, defined as being representative of the absorption coefficient, consists firstly, in the dampening of the amplitude of the signal (the energy received decreases with time) and secondly, in the shifting of the central frequency of the Gaussian spectrum of the received signal towards the low frequencies. In the measurement of an absorption coefficient, the amplitude attenuation is not noted. However, the shift of the central frequency of the spectrum is directly interpreted as being representative of the tissue to be characterized. Finally, the characteristic measured is the capacity of a tissue to shift the central frequency to a greater or a smaller extent, depending on a unit distance between the two points.